Effect of Moisture on Wood Burning Efficiency, Revised – Installment 2
By John E. Laswell, Mechanical Engineer – Aug 2008
Disclosure Statement – the information in this article is mainly the opinion of the author except where websites are shown. There may be errors in the information and errors in any calculations. So, the ideas presented herein should not be used to design a heating system for any home or building. Application of any engineering formulas and principles may be in error. This article is stickily for the reading enjoyment of the curiously minded individual!!
Forward, Hello fellow DIYers. My name is John L. and I live in the hills of scenic, southern Indiana. I have burnt a wood furnace in my basement for 22 years. The wood came from my 38 acre farm, mainly from dead or non-timber grade trees. My wood was cut with a chain saw and hand split for the first nineteen years. I now have a 27 ton hydraulic log splitter. I mainly burn oak, hickory, ash, hard maple and dogwood. For the last three years I have I have been heating one half of my home with a King, 1107J, Blaze King stove. It is wonderful. I have installed it in my dining room. In my opinion this is the best wood stove. If there is any interest, I would write an article about my experience with buying, installing, operating and the pure enjoyment of heating with the “king”.
I am so tired of products that have been “value-engineered” to the point of planned-failure-in-design. How many times have you been asked when you purchase a fairly simple product if you wanted to buy an extended warranty plan? Here is some of the modern philosophy: use it up, burn it up or throw it out when it stops working. I recently bought a pair of trousers where it cost almost as much to alter the length as the cost of the trousers! Something is wrong! I know what it is? Products made in China and cost of labor in USA.
This article comes in three installments:
First – Introduction, Wood Stove Efficiency and Results
Second – Determining The Percent Of Moisture In A Wood Sample, Conclusions, Energy to Cook-off Moisture, Operating the Wood Stove
Third – What Is The Value of Burning Wood?, Creosote and Soot – Formation and Need for Removal, Chimney Fires, Combustion of Wood
Determining The Percent Of Moisture In A Wood Sample
Before I consider another example, I want to present a method for determining the amount of moisture in a piece of wood. For example I had 6 pieces of white oak that weighed 20 pounds and 2 ½ ounces (20.156 lbs). So, I decided to conduct my own experiment! I took my sample piece of white oak and “split” it into 14 smaller pieces which could be easily dried in my oven at about 125 deg F. The initial weight of the log was 3.625 lbs and the final weight of the 14 sticks was 2.25 lbs which was a weight loss of 1.405 lbs. With proportion I then determined what the estimated total water loss would be for all six pieces of white oak: 20.156 / 3.625 x 1.405 = 7.812 lbs of water!! That is almost one gallon of water!!! Would you place 12 pounds of dry wood in your stove and then pour almost 4 quarts of water on top? No! No! No! So, the percentage of water in my arm load of wood was 7.812 / 20.156 = 38.76 %. Looking at Table 1 at a moisture of 40% shows a heating efficiency of 43 % for the great EPA approved stove!
What is wrong with the great EPA stove? I thought that it was rated at 82.5 percent efficiency? Well, nothing is wrong with the EPA stove. I am simply trying to burn “very wet wood”!!!! Even though this white oak had blown down 2 years before, its wood still had a high moisture content because it had not been split and seasoned!! What could I do? By drying the wood down to 10 % moisture, you can see from Table 1 that the thermal efficiency of the great EPA stove could be increased to 73 %. Hello!!!! That is almost double the efficiency of burning my newly split, wet wood. This means, practically, that I would use about one-half the weight of wood if my white oak was well seasoned.
I can tell you friend that large logs take a very long time to season down to 10 to 15 percent moisture. That is one reason why they last so long in the overnight firebox. The lesson to be learned even by “old timers” is that lots of heat and high efficiencies can only be obtained by using seasoned wood (one year after splitting, stacking and covering). It is a crime not to cover dry wood or take a piece of wood covered with snow or ice and place it in a thermally efficient, Phase II EPA approved stove, especially with a catalytic combustor!!!
Strategies for Your Wood Supply
With the coming of Spring a young man’s thoughts turns to love, but with the falling of the leaves, the wise man’s mind tells him to get in his wood supply for Winter. There is also the inherent thought that tree wood will be more dry since the tree sap has returned to its roots. This is somewhat faulty thinking. The moisture in a live tree in the Fall and Winter is still too high for efficient burning and heating. Yes, some trees like Ash can be cut “green”, split and burnt, but not efficiently and not without a lot of smoke and creosote.
Wood will ultimately assume a moisture content level consistent with the relative humidity of its surroundings. It will absorb or lose moisture from or to the atmosphere until the vapor pressure in the wood is equal to the partial pressure in the surrounding atmosphere. Note, it will take 2-3 times the amount of green wood to “heat your dwelling” as well-seasoned wood! Do you understand the last sentence that I just wrote? Let me repeat it; “it will take 2-3 times the amount of green wood to “heat your dwelling” as well-seasoned wood!”.
Do you like cutting, splitting, and hauling lots of wood each Fall? Do you like to have a lazy, smoldering, smokey fire which doesn’t produce much heat unless the draft is really “cranked up”? And you don’t worry about creosote buildup in you chimney or about having a chimney fire? Do you just dump your wood in a pile and fetch the wood into the house as required, be it rain, shine, snow or ice? Do you have an old, inefficient stove. Then, you should not think about burning seasoned wood! Otherwise, a paradigm shift in thinking is required. Have a plan to cut some or all of next year’s wood supply this Fall and Winter. Why procrastinate, you have to take the time to cut the wood sometime, right? But, by cutting and seasoning the wood 6 – 12 months ahead of burning, you only have to cut half the amount, hello again! If you have an old, inefficient stove, then think about upgrading to an EPA Phase II stove which could take your 40% moisture-wood burnt in the old stove which has an efficiency of 26% to a Phase II stove burning 15 % moisture wood which has an efficiency of 59%! That is 2.25 times less wood!
But, what can I do to improve the quality of my wood supply this year? Start cutting wood as soon as possible, even if it is June, July or August. Wood cut in the Summer that is split and stacked properly will loose several percentage points of moisture by December. The rate of moisture-loss does decrease with cooler temperatures. Spit the wood more times than usual to increase the rate of moisture evaporation (remember how I split my white oak sample into 14 pieces and how fast it dried!). Use the finer, split wood to help burn the larger greener wood. Try to select the dead standing trees to harvest first. Ring-cut any live trees early, then the drying leaves will help ‘suck’ the moisture out of the tree. Taking wood into a warmer, dry shelter will help, but? Yes, you say, you just take some wood in by the fire and dry it before you burn it, really. Try checking its percent of moisture content. A professional, dehumidifying, drying kiln takes 35 days to take green oak to 8% moisture. A deliberate wood-seasoning program is the best, least expensive method to provide quality fuel for your stove and for your enjoyment.
Energy to Cook-off Moisture
I think it might be intuitive to think that the percentage of heat of the total charge of wood to cook-off the moisture in the wood is directly related to the percentage of the moisture in the wood. That is the more moisture you have in the wood, then the more wood must be burnt to evaporate that moisture, makes sense! But, what exactly is the relationship. Here is a mathematical proof to demonstrate that intuition and to quantify the relationship.
Let Wwm = the weight of arm load of wood with moisture in pounds.
And m = the percentage of moisture in the wood.
Let Wm = weight of moisture in the wood in pounds.
Thus, Wm = (m / 100) * Wwm
Let Ww = weight of the wood fiber,
then Ww = Wwm – Wm. This means that the weight of the wood fiber is equal to the weight of the charge minus the weight of the moisture in the charge, makes sense.
Let h = heat value for dry wood in BTUs per pound, about 7,000 BTUs per pound of wood.
Let Ch = constant value to evaporate one pound of ice in wood = 1297 BTUs per pound of water.
Then let, k = h / Ch which is assumed to be a constant for each species of wood and is in pounds of water per pound of wood.
Calculating , k = 5.397 pounds of water per pound of wood.
Let, Wwb = weight of dry wood required to evaporate the moisture, pounds.
Thus, Wwb = Wm / k or Wm / Wwb = k = 5.397
Or , Wwb / Wm = 0.1853 or 18.53%
Let Mwb = percent of total charge of wood to evaporate the moisture.
Then the relationship of Mwb and m would be:
Mwb = Wwb / Wwm * 100 and Wwm = 100 Wm / m from above.
Therefore, Mwb = (Wwb / (100 Wm / m)) * 100 = (Wwb * m) / Wm = (Wwb / Wm) * m
Substituting, Mwb = ((Wm / k) / Wm) * m = m / k = m / 5.397 which says that the moisture of the wood, m divided by the constant k = 5.397 will be equal to the percentage of the dry wood required to evaporate the moisture. So, in our example of 20 percent moisture-wood divide by 5.397 is 3.70 percent of the total charge which will be required to cook-off the water.
It is true that this percentage could easily have been calculated by simply taking Wwb divided by Wwm = 0.74 / 20 = 3.7 %. However, knowing the relationship of Mwb = m / k = 0.1853 * m or the percent of wood required to cook-off the moisture is always about 20 percent of the moisture percentage. How easy then to calculate say, 40 percent moisture-wood would require an additional 8 percent (40 x .2) of the wood “charge” to cook-off water. I can then quickly calculate that 48 percent (40 + 8) of the wood “charge” involves the moisture in the wood that does not contribute to heating the intended surroundings. One more example of say 15 percent moisture-wood, would only involve 18 percent (15 + 3) of the wood charge.
In summary, it has been shown that the presences of the moisture in the wood contributes about 5 times more to inefficient burning of the wood as does the heat required to “cook-off” that moisture. The lesson again is to use seasoned wood for fuel with as low a moisture content as is practicable.
Operating the Wood Stove
It is a fact of life that things that become habitual are enjoyed less than the occasional happenstance! So, the wood stove was “laid aside” in late Spring. The ashes were cleaned out, the excess wood and kindling were taken back to the wood shed and the hearth cleaned-up. However, come October with the circus of Fall colors and cooler evenings, your attention turns to a warm jacket, gloves, sock cap, hot chocolate and the wood stove. If you were as wise as the ants, then your wood supply has been seasoning since last Fall or Winter. Your wood is dry and maybe all you have to do is to bring it into the “wood shed”, that is great. Using the wood stove in early Fall or late Spring is a bit tricky because the warmth that feels so good in the morning or evening is unwanted during the warmer day light hours. So, maybe just a little kindling or sticks picked up in the yard are used to take the chill out of the air. A serious fire will not be started for several more days, for an Indian Summer may be just around the corner.
The sign for “the fire” may be frost on the pumpkins, having to breaking a little ice or that the house is just not comfortable even with that warm sweater. It is time to wad-up some paper, place some kindling on top of the paper, top the kindling with some small pieces of wood, open the draft / bypass and strike the match. Don’t let the fire get to hot before controlling the draft and when the fire has a few good coals, start adding some larger pieces of wood. The goal is to establish a good bed of coals before the “main charge” of wood is added to the fire. The “main charge” of wood will be dictated by the time of day, the coolness / coldness of the weather and the temperature of the home. Charging the stove every few hours to produce a “good burn” is more efficient and desirable than “filling her up” and letting the fire smolder. A larger charge can be used if you expect to be away for quite some time. And of course, a larger charge will be used for the overnight heat. As the weather becomes colder, the charges must become larger.
How much wood will you need? It depends on a few factors. Where do you live (part of country)? What kind of a home are you heating? How warm do you want to be? How efficient is your stove? How seasoned is your wood? Heating engineers use a factor known as monthly degree-days which you should be able to acquire from your weather service / station. A degree-day accrues for every degree the average outside temperature is below 65 deg F during a 24 hour period. Degree-days are directly related to the total yearly amount of heat energy you might use if you had a thermostatically-controlled, heating system which a wood stove is not.
For example, Los Angeles – 1391; New York – 5280; Minneapolis – 7977 have these typical degree-days. The value of knowing the degree-days for wood stove users is in predicting the percent of wood that will be used each month of the heating season. The percentages for each month for Bloomington, Indiana (5348 degree-days) for the largest degree-days months are: Nov – 11.7%, Dec – 18.7%, Jan – 21.5 %, Feb – 17.1% & Mar – 13.3 %. For instance, let me say that I use 1/4 chord of wood in November. Then I would expect to use 1/4 chord in March, 1/2 chords in January, 3/8 chords in December and February each and about 3/8 chords for the rest of the heating months for a total of almost 2&1/8 chords of wood. These degree-days are of course averages for the time period from 1971 to 2000. Here is a wood proverb: it is best to have too much seasoned wood than not enough! Enjoy you wood stove heating experience.
Another application of the monthly degree-days factor is to estimate the average, daily high and low temperatures for the heating season. First, the daily degree-days for any month can be calculated by dividing the monthly, degree-days by the number of days in a particular month. So, the number of degrees below 65 deg F in November for Bloomington, IN is [5348 x (11.7 / 100)] / 30 = 20.9 degrees. These degrees should be subtracted from 65 deg F which is the basis for the degree-days factor; therefore, 65 – 20.9 = 44.1 deg F which is the estimated, average temperature for any day in November. Assuming that the difference between the high and low temperature for any one day is 20 deg F and assuming that the high temperature is half of the 20 degrees difference. Then the estimated, high temperature would be 54.4 deg F and the estimated low temperature would be 34.4 deg F for November. I understand that this is a rather simplistic approach for estimating the high and low temperatures, but after all it is estimating the weather!